Silver will tarnish over time. This does not mean there is damage to the key or silver, it is just a chemical reaction. Sulfur has interacted with the silver and created a black/brownish looking patina oxide. To remove it, you will have 2 options:
You can remove it physically by rubbing it off with some kind of cloth or tincture, but this will also remove the silver from the keys and the silver plating will finally be gone.
There are chemical ways of extracting the sulfur, which will keep the silver but will need to be done in combination with an overhaul. This is because it will damage the pads and bumpers making a repadding necessary.
Our advice is either to let it tarnish or if you would like to remove the oxide, do it very carefully.
Nevertheless, you can try to prevent it from happening by putting a so-called silver protection strip
inside your case. In addition, you should always wipe off your fingerprints after you play the instrument and put it back into the case